Adjustable voltage autotransformer



y 1966 G. o. FREDRICKSON ETAL 3,249,850

ADJUSTABLE VOLTAGE AUTOTRANSFORMER Filed Aug. 2, 1963 1N VEN TOR5 ills/2 R. Perri/7s Z5 M arm/Quays 3,249,850 ADJUSTABLE VOLTAGE AUTOTRANSFORMER Gustav O. Fredrickson, Southington, and Allen R. Perrins, Cheshire, Conn, assignors to The Superior Electric Company, Bristol, Conn., a corporation of Connecticut Filed Aug. 2, 1963, Ser. No. 299,529 Claims. (Cl. 323-47) The present invention relates to a transformer for providing an A.C. output voltage that is adjustable in value and more particularly to such a transformer which varies Patented May 3, 1966 ice ' are electrically isolated from each other and spaced a the value of the output voltage by movement of an electrical commutating brush.

One type of transformer that is presently commercially available that provides an adjustable output voltage is an autotransformer that includes a closed magnetic core on which a winding is wound. The winding has similar portions of its exterior turns bared for conducting segments which comprise a commutating surface and mounted for sliding electrical engagement .with the commutating surface is a commutating brush. Thus by connecting the winding to a source of alternating current, the brush may be moved on the commutating surface to vary or adjust the value of the output voltage. However, insuch a structure, wherein the width of the brush contacting the commutating surface is wider than the width of one segment, the brush generally spans two segments which produces a circulating current caused by the turn between the two segments short-circuited by the brush and this circulating current flows in the brush in addition to the output current. The value of the circulating current is limited by a resistance, which is mostly the contact resistance occurring atthe interface of the brush and the commutating surface and as such produces a localized heating at'the interface. To provide durability and reliability and prevent accelerated deterioration and/or malfunctioning of the autotransforme-r the heating and hence temperature at the interface must be restricted which in turn serves to limit the amount of current which the autotr-ansformer may control.

It is accordingly an object of the present invention to provide an adjustable voltage autotransformer of the above-noted type which is capable of controlling a larger value of output current than has heretofore been'possible in similar size autotransformers without a decrease in durability and reliability.

Another object of the present invention is to increase the current carrying capacity of an autotransformer but yet maintain the localized heating at a brush within theacceptable limits. I

A further object of the present invention is to provide an adjustable voltage autotr-ansforme-r that achieves the above objects for retaining the continuity of voltage adjustment throughout the maximum and minimum values of the autotransformer with a minimum of alteration in the presently available design of said autotransformers.

In carrying out the present invention, there is provided an autotransformer of the above set forth type which in the embodiment hereinafter specifically disclosed includes a winding Wound on a closed magnetic core with a commutating surface formed on the exterior turns of the windings by each turn constituting an independent segment in the commutating surface. A brush structure is mounted for sliding electrical engagement on the commutating surface on a rotatable shaft that may be manipulated to change the location of the brush on the commutating surface and hence effect the adjustment in value of the output voltage.

More specifically, the brush structure, rather than consisting of just one brush, consists of at least two that few segments apart. Thus, each brush functions as if it were the only brush on the commutating surface and thus the condition inherent with each brush is made to be substantially independent of the other brush. Accordingly, by the use of this brush structure the localized heating and the current flow through each brush may remain the same as in a single brush autotransformer but yet by the use of two brushes a substantial increase in the current handling capacity over a single brush autotransformer is achieved.

In the above brush structure, each brush may attempt a to handle more current than is advantageous therefor with the other handling less than it is capable, and thus onemay exceed its determined localized heating and current by each brush and in the embodiment herein specifically 1 described, the ratio is 1 to 1 so that, each brush carries the same amount of output current. Thus only one half of the total output current is accordingly passed through each brush.

Other features and advantages will hereinafter appear.

In the drawing: 1 7

FIGURE 1 is an electrical schematic diagram of the present invention.

FIG. 2 is a plan view of. an autotransformer having the present invention.

FIG. 3 is a partial elevation, partly in section thereof.

FIG. 4 is a diagrammatic section taken on the line 44 of FIG. 3.

FIG..5 is a detail of a radiator carrying the brushes.

FIG. 6 is an enlarged view of the brush structure.

FIG. 7 is a sectional view taken on the line 7-7. of FIG. 6.

Referring to the drawing, the adjustable voltage autotransformer is generally indicated by the reference numeral 10 and as specifically shown includes an annular core 11 formed by spirally winding a strip of magnetic iron on which are positioned protective elements, generally indicated by the reference numeral 12. Toroidally wound on the core 11 and over theprotective elements 12 is a length of insulation covered, electrical conducting wire that forms a winding 13, the winding extending as shown in FIG. 2 substantially 330 of the annular core '11. A commutating surface 14 is formed by removing from a similar portion of each of the outer turns of the Winding the insulation thereon and making a secant in circular wire so that each outer turn has a flat surface or segment, each being indicated by the reference numeral 15. The segments 15 define the commutating surface and preferably the commutating surface extends throughout the length of the winding, in the structure herein described, with each segment 15 being substantially identical in Width.

The core and winding are supported in a frame, generally indicated by the reference numeral 16, that includes a plate 17 having a bearing aperture in which a shaft 18 is rotatably mounted. The core is held to the frame by feet 19. While only an upper bearing aperture plate 17 is shown, it willbe appreciated that there is an identical bearing plate on the other side of the core with the plates being secured together by bolts 16a as is well known in the art.

Mounted on the shaft 18 for rotation therewith is a radiator 20 consisting of a first radiator piece 21 and a second radiator piece 22 symmetrical to the first piece.

0 The pieces 21 and 22 are mounted on the shaft 1 8 and shaft and abut a bolt 24 and insulating connecting strip 2 5. If desired, insulation may be positioned within the joint 26 therebe-tween as by insulation 26a.

The radiator piece 21 carries a brush, generally indicated by the reference numeral 27 by means of a holder 28 having an car 2 9 through which a screw 30 passes to mechanically fasten and electrically connect the holder to the piece 21. The brush 27 herein is shown as being formed in two sections 27a and 27b each independently movable with respect to each other and urged by springs 31a and 31b downwardly against the commutating surface '14. Connecting wires 32a and 32b form a connection from the brush to the holder 28. Thus current is conductibie from the brush section 27a through the conductor 32a to the piece 21 for the brush section 27b to the conductor 32b to the piece 21.

The radiator piece 22 has a similar brush construction 33 that is identical though symmetrical to the brush 27 and thus includes brush sections 33a and 33b together with conductors 34a and 34b for conducting current from the brush to the radiator piece 22. A brush holder 35 fastens the brushes to the radiator piece 22 and similarly provides for conduction thereto.

To assure electrical isolation between the brushes there is provided an insulating piece 36 positioned between the brushes 27 and 33. While only one brush section may be employed for each section by having two sections, each individually movable vertically of the commutating surface, a more positive electrical contact with the commutating surface is assured.

Referring to the schematic diagram FIG. 1 the winding '13 is shown as having input connections 37 and 38 thereto, the latter being connectibie to a source of alternating current. The brushes 27 and 33 are also schematically shown and indicated. The output of the adjustable voltage autotransformer where the adjusted output voltage appears is between a terminal 39 and a terminal 40, the terminal 39 being common with the input connection 37.

In carrying out the present invention in order to prevent one brush from carrying more output current than its maximum tolerable value, a desired ratio of output current between the two brushes is provided by reactor means 41. In the specific embodiment shown, while the ratio between the brushes of the output current may be any particular ratio desired, it is herein shown as dividing the output current in half so that the output current in each brush will be substantially equal. The reactor means 41 consists of an annular closed magnetic core 42 on which is wound a first coil 43 and a second coil 44. One end of the .coil 43 is connected to the brush 27 as is an end of the coil 44 to the brush 33, while the output terminal 40 is connected to a common junction 45 of the two coils. It will be appreciated that with this construction that the output current to the terminal 40 has one half flowing through the coil 43 and its other half flowing through the coil 44. The coils 43 and 44 if it is desired to have the output current divided evenly between the brushes 27 and 33, have the same number of turns but are wound in the same direction on the core 42 but as the output current flows in opposite directions in the coils, the ampere turns produced by each coil in the core cancel each other. Thus if one brush should tend to carry more current than the other brush, the reactor means will introduce an impedance in the one brush coil which decreases the current therein and thus prevents the output currents from becoming unbalanced. Also, the reactor means 41 has sufiicient impedance to limit the no load .current in the windings 43 and 44 caused by the voltage difference existing between the brushes 27 and 33 to a negligible value.

While the reactor means 41 may be physically mounted with respect to the autotransformer in many ways, as for example on a separate frame, one embodiment of the physical structure thereof is shown in the drawing wherein the core 42 being annular and formed by spirally winding a strip of magnetic material is supported in any convenient manner on the bottom plate (not shown) to encircle the shaft. The connection between the brush 27 and the end 43 is by a lea-d 46 that has a few turns encircling the shaft with the end, for example, being connected to any part of the radiator piece 21 as the screw 30. In addition, another wire 47 extends from the radiator piece 22 as for example the corresponding screw of the holder 35 to the other end 44 of the coil after having a few turns encircle the shaft. The coil as shown consists merely of a few turns of wire wound on the core 42 and is center-tapped as at 45 to form the common junction 40 with a lead 48 extending therefrom. By wrapping the leads 46 and 47 around the shaft two or three turns, the radiator 20 may rotate within its 330 arc with respect to the reactor means 41. Of course, it will be understood that if desired sliding or slip ring contacts may be employed.

Referring to FIG. 4, there is shown a diagrammatical representation of a cross-section of the two brushes 27 and 33 on the commutating surface. Each brush has a face, indicated by the reference numerals 49 and 50 that contact the segments and the width of each face is wider than one segment 15 and thus in all positions of the brushes they contact at least one segment thereby providing continuity of the output voltage as the brushes are moved and/or located.

According to the present invention the brushes 27 and 33 are separated from each other for at least one segment .and in the instant embodimentdisclosed for four segments. With this construction it will be appreciated that the localized heating developed at the surfaces 49 and 50 accordingly occurs at two spaced apart locations on the coil.

The winding 13 has the input connections 37 and 38 connected intermediate the ends of the windings as shown in FIG. 1. The range of the output voltage to be adjusted is preferably from a minimum of zero to a maximum which may be that of the voltage existing between input leads 37 and 38. In order to achieve such a range of adjustment, the input connection 37 is connected to a segment which is located two turns from the end of the commutating surface While the tap 38 is located inwardly two turns from the top of the commutating surface. By this arrangement the number of segments between the segments connected to the turns 37 and 38 plus the four segments located between the two brushes equal the total number of segments in the commutating surface. While the embodiment described subdivides the number of segments between the brushes and places one half at each end of the commutating surface, it will be appreciated that if a different ratio of output current between the brushes is desired that this ratio of the segments will change in the same proportion.

The number of segments between the brushes has been disclosed as being four though it will be understood that the invention is not limited thereto. While increasing the number of segments between brushes will more widely separate the localized heating at each brush, it will also require a larger reactor means because of the increased voltage difference between the two brushes in addition to increasing the segments at the end of the commutating surface.

In addition to separating the supports where localized heating occurs on the commutating surface, it will be appreciated that the brushes moreover, by being mechanically connected to the two piece radiator, one brush to each piece, that each of the pieces serves as a heat conducting member substantially solely for its respective brush and thus aids in conducting the heat away from the localized heating area. At least the part of the brushes forming the faces 49 and 50 is preferably formed of a carbon compound as is well known in the art.

it will accordingly be appreciated that there has been disclosed an adjustable voltage autotransformer which has a current handling capacity greater than has heretofore been achieved by similar sized structure. The present invention achieves this ability by the use of at least two commutating brushes, each of which is electrically isolated from the other, separated by at least one segment of the commutating surface and has a width of contact greater than the width of one segment. The current from the brushes is maintained in the ratio desired by the utilization of reactor means which opposes any change in the ratio that the output current is divided between the brushes and thus maintains the ratio at that initially determined.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

We claim:

1. An adjustable voltage autotransformer comprising a closed magnetic core, a winding wound on said core, a commutating surface consisting of individual segments of substantially constant width formed on said winding, a first brush and a second brush, each of said brushes being in electrical engagement with said surface and being wider than one segment, means mounting the brushes for movement on said surface with a separation therebetween of at least one segment, reactor means connected to the brushes for maintaining the ratio of output current through the brushes substantially constant, and output means including a connection to the reactor means.

2. An adjustable voltage autotransformer comprising a closed magnetic core, a winding wound on said core, a commutating surface consisting of individual segments of substantially constant width formed on said winding, 7

a first brush and a second brush, each of said brushes mounting the brushes for movement on said surface bebeing in electrical engagement with said surface and being wider than one segment, means mounting the brushes for movement on said surface with a separation therebetween of at least one segment, reactor means connected to the brushes for maintaining the ratio of output current through the brushes substantially constant, said reactor means including a core having one coil connected to one brush and another coil connected to the other brush with the ampere turn flux in each coil being substantially equal. and opposite, and output means including a connection common to both coils.

3. The invention as defined in claim 2 in which the current in each brush is substantially equal and both coils have substantially the same number of turns.

4. An adjustable voltage autotransforrner for providing an output voltage adjustable between a maximum and a minimum comprising a closed magnetic core, a winding wound on said core, a commutating surface consisting of individual segments of substantially constant width formed on said winding, a first brush and a second brush, each of said brushes being in electrical engagement with said surface and being wider than one segment, means mounting the brushes for movement on said surface between a maximum and a minimum voltage position with a separation therebetween of at least one segment, input connections to said winding with the input connection to the minimum end of the winding being located inwardly of the minimum voltage position, reactor means connected to the brushes for maintaining the ratio of output current tween a maximum and a minimum voltage position with a separation therebetween of at least one segment, input connections to said winding with the input connections being located to have lesser segments of the winding therebetween than between the maximum and minimum voltage positions, reactor means connected to the brushes for maintaining the ratio of output current through the brushes substantially constant, and output means including a connection to the reactor means.

6. The invention as defined in claim 5 in which the ditterence in the number of segments between the input connections and the voltage positions equals the number of segments by which the brushes are separated.

7. An adjustable voltage autotransformer for providing an output voltage adjustable between a maximum and a minimum comprising a closed magnetic core, a winding wound on said core, a commutating surface consisting of individual segments of substantially constant width formed on said winding, a first brush and a second brush, each of said brushes being in electrical engagement with said surface and being wider than one segment, means mounting the brushes for movement on said surface between a maximum and a minimum voltage position with a separation therebetween of at least one segment, input connections to said winding with the input connections being located to have lesser segments of the winding therebetween than between the maximum and minimum voltage positions, reactor means connected to the brushes for maintaining the ratio of output current through the brushes substantially constant, said reactor means includjacent voltage position a distance equal to half the seg- I in electricalengagement-with said surface and being wider than one segment, means mounting the brushes for movement on said surface with a separation therebetween of at least one segment, said means including a rotatable shaft with a two piece radiator mounted on said shaft with the pieces being mechanically connected but electrically isolated and a brush mounted on each piece in electrical engagement therewith, reactor means connected to the brushes for maintaining the ratio of output current through the brushes substantially constant, and output means including a connection to the reactor means.

10. An adjustable voltage autotran-sformer for pro viding an output volt-age adjustable between a maximum and a minimum comprising a closed magnetic core, a winding wound on said core, a commutating surface consisting of individual segments of substantially constant width formed on said winding, a first brush and a second brush, each of said brushes being in electrical engagement with said surface and being wider than one segment, means mounting the brushes for movement on said surface between a maximum and a minimum voltage position with a separation therebetween of at least one segment, said means including a rotatable shaft with a two piece radiator mounted on said shaft with the piece-s being mechanically connected but electrically isolated and a brush mounted on each piece in electrical engagement therewith, input connectionsto said winding with the input connections being located to have lesser segments of the 7 8 winding thercbetween than between the maximum and References Cited y t a minimum voltage positions, reactor means connected to UNITED STATES PATENTS the brushes for maintaining the ratio of output current 1631955 6/1927 Bergman 336 146 X through the brushes substantially constant, said reacto 2/1959 Deniau 323 43 5 means including a core having one coil connected to onc 5 :9 9:92 3 1950 Smiley 336 148 brush and another coil connected to the other brush with 3 1 5 94 5 19 5 lb 336 148 X the ampere turn flux in each coil being substantially equal and opposite, and output means including a connection LLOYD MCCOLLUM Pnmary Exammer' to the reactor means. W. E. RAY, Assistant Examiner. 

1. AN ADJUSTABLE VOLTAGE AUTOTRANSFORMER COMPRISING A CLOSED MAGNETIC CORE, A WINDING WOUND ON SAID CORE, A COMMUTATING SURFACE CONSISTING OF INDIVIDUAL SEGMENTS OF SUBSTANTIALLY CONSTANT WIDTH FORMED ON SAID WINDING, A FIRST BRUSH AND A SECOND BRUSH, EACH OF SAID WINDING, BEING IN ELECTRICAL ENGAGEMENT WITH SAID SURFACE AND BEING WIDER THAN ONE SEGMENT, MEANS MOUNTING THE BRUSHES FOR MOVEMENT ON SAID SURFACE WITH A SEPARATION THEREBETWEEN OF AT LEAST ONE SEGMENT, REACTOR MEANS CONNECTED TO THE BRUSHES FOR MAINTAINING THE RATIO OF OUTPUT CURRENT THROUGH THE BRUSHES SUBSTANTIALLY CONSTANT, AND OUTPUT MEANS INCLUDING A CONNECTION TO THE REACTOR MEANS. 